1. Field of the Invention
This invention relates to a position measuring apparatus using a precise positioning apparatus, and in particular to an apparatus for detecting a mark provided on a substrate which is an object to be positioned and measuring the position of the substrate.
2. Related Background Art
As an apparatus for two-dimensionally positioning a substrate such as a semiconductor wafer or liquid crystal, there is known a step-and-repeat type projection exposure apparatus. The apparatus of this type is such that a pattern formed on a reticle is projected onto a wafer by a projection lens and only a local area on the wafer is exposed, whereafter the wafer is caused to effect stepping to expose an adjacent local area. For this reason, the wafer is placed on a two-dimensionally moved stage. Now, usually, the wafer is exposed with several layers of circuit patterns being superposed thereon and therefore, it becomes necessary to align the reticle and the local area on the wafer. Several methods of alignment have been put into practical use and used in the field of the manufacture of semiconductors. As one of such alignment systems, it is known to use an apparatus disclosed, for example, in U.S. Pat. No. 4,677,301. FIGS. 1A and 1B of the accompanying drawings illustrate an alignment system using a prior-art apparatus. In FIG. 1A, a wafer 8 is placed on a two-dimensionally moved stage 1, and the position of the stage 1 is sequentially measured by a laser light wave interference type measuring machine 2 (hereinafter referred to as the interferometer) with a resolving power of the order of 0.02 .mu.m. A fiducial mark 5 of the same shape as a mark WM preformed on the wafer 8 is fixed on the stage 1. A reticle 4 having a pattern to be exposed is set at a predetermined location above a projection lens 3. The projection lens 3 serves to project the pattern of the reticle 4 onto the photoresist layer of the wafer 8.
Now, in this apparatus, there are provided two alignment systems, one of which is a TTL alignment system for detecting the mark WM or the fiducial mark 5 of the wafer 8 from between the reticle 4 and the projection lens 3 through the projection lens 3 and the other is a TTR (through the reticle) alignment system 10 for detecting a mark RM provided on the reticle 4 and the fiducial mark 5 at a time. The TTL alignment system has a laser light transmitting system 7 for outputting a laser light (He-Ne or the like) for alignment and forming a sheet-like beam on the wafer 8 or on the fiducial mark 5, and a light-receiving system 6 for detecting light information from the mark. This light-receiving system 6, with the interferometer 2, is used for the detection of the position of the mark. The details of this mark detecting operation are disclosed in the aforementioned U.S. Patent and therefore need not be described herein.
In such a construction, the stage 1 is first moved, and then is positioned so that the fiducial mark 5 and the mark RM of the reticle 4 are detected in a predetermined positional relation by the TTR alignment system 10. The position of the stage 1 when this positioning is completed is detected by the interferometer 2.
Subsequently, the stage 1 is moved so that the fiducial mark 5 is scanned relative to the sheet-like beam from the laser light transmitting system 7 of the TTL alignment system, and at this time, the light information from the fiducial mark 5 is detected by the light-receiving system 6, and the position of the stage 1 in which the sheet-like beam and the fiducial mark 5 coincide accurately with each other is detected by the interferometer 2. The stage 1 is further moved, and then is positioned at a position in which the mark WM of the wafer 8 is detected by the sheet-like beam of the TTL alignment system as shown in FIG. 1B. Likewise, the position of the stage 1 when the mark WM and the sheet-like beam have accurately coincided with each other is detected by the light-receiving system 6 and the interferometer 2.
The relative positional relation between the reticle 4 and the fiducial mark 5 and the relative positional relation between the fiducial mark 5 and the wafer 8 are found by the above-described operation, and after all, the relative position of the reticle 4 and the wafer 8 is indirectly found.
Accordingly, during the actual exposure of the step-and-repeat type, the wafer 8 can be aligned at a predetermined superposition exposure position on the basis of only the position information of the interferometer 2.
In the above-described prior-art system, the position of the wafer 8 relative to the reticle 4 is detected through the operation of measuring the position of the stage 1 and therefore, when yawing occurs to the stage 1, there arises the problem that an error is included in the position detection. Yawing means the rotation of the stage 1 on a two-dimensional coordinates system by the x-axis and the y-axis. The prior-art interferometer 2 only measures the coordinates value of the stage 1 along the x-axis and the y-axis orthogonal to each other and therefore, even if a minute rotational deviation occurred to the stage 1 in a plane containing the x-axis and the y-axis, it could not detect the minute rotational deviation. This has led to the problem that when the amount of yawing differs due to the difference in the position of the stage 1 between during the detection of the fiducial mark 5 and during the detection of the mark WM of the wafer, a so-called Abbe error occurs.